In chambers for CVD, PVD, or etching of articles, which articles are hereinafter generically referred to as wafers, it is common to support the wafer on a chuck. RF bias energy can be supplied by the chuck supporting a wafer to provide uniform electrical coupling of the bias energy through the wafer to the plasma. The wafer is also thermally coupled to the chuck to minimize the possibility of overheating of the wafer.
To overcome problems of poor thermal and electrical coupling between the wafer and the chuck in the low pressure or vacuum environment during processing, the wafer is usually clamped to the chuck by means of a mechanical clamping systems or an electrostatic chuck which employs the attractive coulomb force between oppositely charged surfaces to clamp the wafer to the chuck. Because wafer processing usually occurs in a very low pressure environment, it has been found to be desirable to introduce a gas such as, for example, helium, hydrogen, or nitrogen between the wafer and the chuck to enhance thermal contact and heat transfer. U.S. Pat. Nos. 5,103,367, 5,160,152, and 5,350,479, for example, disclose the use of a gas between the wafer and the chuck for improving thermal transfer. Such chucks typically have cooling jackets for cooling a wafer through thermal contact with the chuck. In addition, co-pending U.S. patent application Ser. No. 08/401,524 discloses an electrostatic chuck having heater electrodes provided therein that can be used to compensate for plasma non-uniformity and/or edge effects and to provide temperature control during processing.
It has proven difficult to accurately monitor wafer temperatures during processing and, accordingly, control of wafer temperature suffers. Infrared pyrometers are not suited for measuring wafer temperatures below 300.degree. C due to transparency of the silicon wafers at such temperatures. Spring loaded contact probes with phosphor applied on them have been used to contact back sides of wafers for in-situ backside wafer temperature measurements, but this technique has not proven to be reliable in a production environment. It is, accordingly, desirable to provide a method and apparatus for reliably monitoring and controlling wafer temperature during processing.